Paper-making machines include press sections positioned before the paper-making machine's dryer section. In the press section, the paper web is drained of moisture (also referred to as dewatering). The drainage rate of the paper web is proportional to the viscosity and surface tension of the trapped water. Increasing the web temperature decreases the water viscosity and surface tension and improves the pressing process. Also increasing the dewatering rate at the press section of the paper-making machine decreases the moisture content of the web before the web enters the dryer section, thereby reducing the energy or time needed to further dewater the web in the dryer section. Therefore, it is common to apply steam to a paper web prior to the sheet entering the press section using the steam to heat the web.
Steam is applied to the web at spaced increments across the paper-making machine. At the dry end of the machine (following the dryer section), the web is passed through a calendar stack. The surface finish and thickness (or caliper) of the web is affected by the moisture and temperature profiles of the web. Therefore the application of steam to the web will influence both the moisture and temperature of the sheet, and at the dry end of the machine, the caliper and surface finish qualities of the web.
The press section of a paper-making machine typically raises the solidity of the web by 20% to 50%. The steam is typically provided by a steam box mounted adjacent to the press roll either before or in the press section to condense steam onto the surface of the web. The design of the steam box is critical to the efficiency of steam usage; the steam box's ability to profile the moisture profile across the web; and the steam box's ability to stay clean. If the steam box cannot remain clean the blockage will reduce its ability to provide sufficient steam to influence the web temperature to provide greater dewatering, and its ability to apply different levels of steam across the web to permit the press to level the moisture profile of the web.
Related art includes U.S. Pat. No. 6,264,795 to Hamel; U.S. Pat. No. 6,408,534 to Alen et al.; and U.S. Pat. No. 5,711,087 to Pazdera.
One disadvantage of prior art steam boxes are that as they are installed in the press section against a roll face they are subject to stock being flung from the roll surface onto the steam box face (i.e. diffuser or screen plate) due to centrifugal forces. Once the face of the steam box is blinded by stock, this blockage may substantially degrade the steam box's ability to deliver steam.